Cyclic phosphonitrilic chloride trimer (hereinafter "trimer") can be used to make high molecular weight substantially linear polyphosphonitrilic chloride by heating the trimer at about 200.degree.-300.degree. C. for 12-24 hours. Unless the trimer is free of certain impurities, the polymerization will lead to a brown-black polymer and will prematurely cross-link to form a useless high gel polymer which will not dissolve in solvents such as tetrahydrofuran, cyclohexane or toluene and thus cannot be reacted with substituting agents to produce the more useful substituted polyphosphazenes.
Several methods are available for making polymer-grade trimer. One such method is described in Adams et al., U.S. Pat. No. 4,603,638. This involves distilling solvent from a solution of the crude trimer to form a hot concentrate and mixing the hot concentrate with water to crystallize trimer in a water-solvent mixture. The crystallized trimer is then distilled.
In Klobucar, application Ser. No. 658,307 filed Oct. 5, 1984, another procedure is described in which a trimer solution is distilled to remove a portion of the solvent leaving a hot concentrate which is cooled to crystallize trimer. The trimer is separated and distilled.
A very effective process for making polymer-grade trimer is described in Adams et al., U.S. Pat. No. 4,605,539. In this process the crude trimer is made in an inert solvent such as monochlorobenzene by reacting ammonium chloride with phosphorus pentachloride. The resultant crude solution is water washed. The washed solution is heated to distill out a portion of the solvent and then cooled to crystallize trimer which is then distilled to give polymer-grade trimer.
More recent research has found that the reaction of ammonium chloride with phosphorus pentachloride is promoted by including a nitrogenous base such as pyridine, Sulkowski et al., Chem. Stos. 26 (2) p. 245-252 (1982). In their research, pyridine was added to a ground mixture of phosphorus pentachloride and ammonium chloride causing a strong exothermic reaction rising rapidly to 180.degree. C. with boiling. The process can be carried out in an inert solvent such as sym-tetrachloroethane.
Tanino et al., U.S. Pat. No. 4,567,028 describe a related procedure in which phosphorus pentachloride and ammonium chloride are reacted in an inert solvent in the presence of a catalytic amount of pyridine or an alkyl pyridine and a polyvalent metal compound.
Allcock et al., application Ser. No. 756,799 filed July 19, 1985 describe a method of controlling the strong exotherm reported by Sulkowski et al. Allcock et al. first react PCl.sub.5 with pyridine to form a complex and then in a second stage react the complex with ammonium chloride to form trimer in high yield under controllable conditions.
Sulzer et al., application Ser. No. 798,910 filed Nov. 18, 1985 describe a very effective way to make trimer in which ammonium chloride and phosphorus pentachloride are reacted in a molten pyridine-HCl complex. The product extracts into a separate solvent phase from which it is recovered. The pyridine-HCl remains as a separate phase and can be recycled.
Although the more recent procedure involving the use of pyridine offers much faster reaction rates and high yields of trimer, they do seem to complicate the purification procedures. For example, trimer made by one of the pyridine routes is not consistently converted to polymer-grade trimer by any of the known purification procedures. Thus a need exists for a purification process that will consistently convert crude trimer made by a process involving the use of pyridine or a substituted pyridine to a polymer-grade trimer.